A plain old telephone system (POTS) is retronym for a voice-grade telephone service typically used in residential homes and small business. The interface to a POTS phone is typically two wires. The tip line is the ground side (positive) of the telephone circuit. The ring line is the battery (negative) side of the telephone circuit. In the United States, the ring line carries −48 volts of DC voltage when in the hook (idle) state. To ring the phone, about 90 volts of 20 Hz AC current is superimposed over the DC voltage on the ring line.
A phone off-hook creates a DC signal path between the tip line and the ring line, dropping the voltage from the ring line to the tip line to about −3 to −9 volts and drawing about 15 to 20 milliamps at a DC resistance of about 180 ohms. Voice on a telephone network is digitized at 8 kHz sampling rate. Effective transmission across telephone lines occurs essentially between 180 Hz to 3.2 kHz range which is sufficient for speech intelligibility while allowing multiplexing of many calls over coax and twisted pair.
Mobile phones often have a headphone jack which allow a headset to be plugged in. The headset jack typically use three lines: a microphone line, a speaker line and a ground line.